Method and apparatus for radio receiving



Oct. 27, 1942. s, Y, WHITE METHOD AND APPARATUS FOR RADIO RECEIVING Filed Feb. 29, 1940 3 Sheets-Sheet l Sidney Y W/v/'fe BY Qta-W94 M ATTO RN EYS Oct. 27, 1942. s, Y. wHrrE METHOD AND APPARATUS FOR RADIO RECEIVING Filed Feb. 29, 1940- 3 Sheets-Sheet 2 Isl t A M f A l l V I m M /Q x W QU .Fx o bwm M S I W QQ w ATTORNEYS Oct. 27, 1942. s. Y` WHITE 2,300,081

METHOD AND APPARATUS FOR RADIO RECEIVING Filed Feb. 29, 1940 3 Sheets-Sheet 3 Annul Sl. Em

ATTORNEYS Patented ct. 27, 1942 TES PTET OFFlCE METHOD AND APPARATUS FOR RADIO RECEIVING 10 Claims.

This invention relates to a radio receiver adapted to receive signal communication from a plurality of transmitting stations, and in which means is provided which is actuated by impulses sent from the transmitting station, for the purpose of identifying the particular station which is transmitting and for permitting the transmitting station to communicate with a desired group of receivers or with an individual receiver of the group. This invention is in the nature of an improvement on the invention disclosed and claimed in application Serial No. 321,378 led February 29, 1940, in my name. In said application a radio receiver is disclosed which is especially adapted for the reception of carrier waves having a frequency higher than 20 megacycles, and wherein inertialess scanning means are provided for rapidly scanning a portion of the spectrum in which a tone modulated carrier is expected to occur and for causing the receiver to automatically lock on such a carrier. It is desirable in connection with such a receiver to provide an indicating device, such as an alarm, which will notify the operator that a carrier has been locked on and the calling party is ready to speak to him. It is an object of this invention to provide an alarm device which will give an audible signal to the operator when the receiver has locked on a carrier. The alarm device is also operative to indicate the reception of a carrier by a scanning receiver which is provided with a director device for causing it to lock on the rst carrier encountered, such a receiver being disclosed in patent application Serial No. 321,377, iiled February 29, 1940, in my name.

Where it is desired to establish two-Way communication between a plurality of stations such as A, B, C, D, etc, each provided with its own radio transmitter, and a receiver of the type above described, a call may be transmitted from station A directed to station B. If B is to set up a communication channel with A, he must obviously know who is calling so that he may arrange for the transmission of a tone frequency which As receiver will lock on. It is another object of the present invention to provide a radio receiver of the scanning type with a plurality of indicating devices which are individual to the other transmitting stations of the group and which indicate the particular station to which the receiver has been automatically tuned. In accordance with a preferred embodiment of the invention, this indicating device comprises a series of electric lights on which are painted or otherwise marked the station designation, such as A, B, C, D, etc., and which are lighted in accordance with the station on which the receiver is locked. 'Upon B nding that A is calling, he may then set up a transmission modulated by the identifying tone upon which As receiver only will lock and the indicating light marked B on As receiver Will light, indicating that the circuit is complete and that he can commence talking It is only after Bs receiver has locked on As carrier that energy impulses may be transmitted by station A to operate the indicator of Bs receiver. Diierent forms of station identifying impulses might be used to operate the indicator as, for example, a selective audio tone in addition to the already existing tone which causes the locking action. It is a further object of the invention to operate the indicator by means of energy pulses in the form of a series of dots. The number of dots will determine the identiiication of the transmitting station, for example, station A would transmit one dot, station B two dots, station C three dots, etc.

A still further object of the invention is to provide a radio receiver with means whereby communication may be selectively established With all the receivers in a group, with a limited number of such receivers or with only an individual receiver of the group. Considering, for example, a group of 100 receivers, all could be made to lock on the general call tone frequency of, for example, 5000 cycles, and by means of a step-by-step relay having a contact arm movable over a plurality of contact points, the contact arm may be moved to contact number l by transmitting one dot. By providing a headphone 0r loudspeaker connection to number l contact and the audio output terminals of each receiver of the group, communication could be had with all the receivers by transmitting the 5000 cycle modulated carrier and after a short interval of, for example, one second, transmitting one dot. Each receiver is preferably also provided with means which cause it to lock on a carrier modulated by another tone frequency. For example, a sub-group of 10 receivers of the entire group of 100 could be made to lock on a tone frequency of 7000 cycles, and by connecting the headphones of the receivers to different contact points, communication could be had with any desired receiver of the group of l0. For example, contact could be established with receiver A by sending two dots, with receiver B by sending three dots, etc. While a total group of 100 receivers has been referred to, it Will be understood that the invention is adapted for use with a considerably greater number of sub-groups of receivers, and practically an unlimited number of receivers in each sub-group.

For a better understanding of the invention reference is made to the following description taken in connection with the accompanying drawings in which:

Fig. 1 is a schematic circuit diagram in block form of the receiver portion of a radio receiving system embodying the invention;

Figs. 2 and 3 are schematic circuit diagrams of a radio receiving system embodying the invention; and

Fig. 4 is a schematic circuit diagram of a radio receiver adapted for use with the invention.

Referring to Figure 1, the receiver shown comprises an antenna I connected to the tunable input circuit 2 of a radio frequency amplifier 3, the circuit 2 being permeability tuned over a band of frequencies as indicated at 4.

It is found that the ordinary hydrogen reduced iron cores are not at all suitable for permeability tuning at frequencies of the order of 100 megacycles. However, it has been found that ferromagnetic .cores known under the trade name Aladdinite are very satisfactory. These cores are made from a synthetically produced ferromagnetic mass powder consisting substantially Wholly of magnetic oxide of iron in the form of minute particles, substantially all of which, as they appear under a microscope, are of generally rounded form. These particles are preferably molded into cores by mixing with several per cent of Bakelite as a binder.

The output circuit of amplifier 3 feeds in turn a first detector or mixing device 5, an intermediate frequency amplifier 6, a second detector 1, audio frequency amplier V8, band pass filter 9, and a signal reproducing device I0, herein illustrated as ear phones, but it will be understood that this device is of a suitable type to reproduce any desired type of signal. If desired, the output level of the receiver may be maintained at a substantially constant Volume level by an automatic volume control device II which may be of any known type. The amplier 3, mixing device 5, intermediate, frequency amplifier it, detector "I, and audio amplifier 8 maybe of any known type as used in superheterodyne receivers for the reception of modulated carrier waves. The oscillator frequency is supplied to the mixing device by an oscillator I2, whose frequency is controlled by a control tube I3 which is subject to the control of a sweep oscillator I4 preferably generating a low frequency, such as a half cycle per second. The control tube I3 is also subject to the control of a director device I5 which is in turn controlled by a switch device I6 operated by one of the tone filters I'I, I'I which are connectedy in parallel with the audio frequency amplifier 8. The detailed construction of the amplifier 8, oscillators I2 and I4, control tube I3, director I5, switch I5, and tone filters II, I'I will be referred to later herein.

In the operation of this receiver, the frequency of the oscillator I2 is automatically controlled over a range of frequencies by the control tube I3 through the action of the sweep oscillator I4. Upon encountering a tone modulated carrier which forms a beat frequency with that of oscillator I2 which lies within the acceptance band of the intermediate frequency amplifier 6, and to which the tone filter, for example the filter II, is responsive, the switch l5 is operated which causes the director device I5 to overcome th action of the sweep oscillator I4 and maintain the frequency of the oscillator I2 at a constantJ value, thus locking the receiver on the carrier. The receiver will naturally remain locked on this carrier until the modulating frequency to which the tone filter I1 is responsive disappears or until the carrier disappears, in which event the sweep oscillator I4 again takes control of the oscillator I2 and the scanning action of the receiver is resumed.

As described in the aforesaid application, the tone lter I'I of all the receivers may be made resonant to a common frequency, as for example 5000 cycles, so that upon the transmission of a carrier frequency modulated with this frequency, all the receivers of a group will lock on this carrier frequency, whereupon the operator at the transmitting station may communicate with every receiver in the group. The tone filter I'l' is made resonant to a frequency which is individual to only one receiver of the group, so that the operator at the transmitting station rby modulating his carrier with this particular tone frequency may establish contact with that particular receiver of the group.

Referring now to Figure 2, the heretofore described receiver of Figure 1 is indicated by the reference character R, the output of the receiver being connected by a transformer I8 to a tone filter I9, which is in turn connected to the coil 20 of a relay 2I, through a rectifier 22, a suitable resistor 23 being shuntecl across the terminals of the relay winding. The armature 24, upon the operation of relay 2I, serves to close a circuit through the battery 25 and the coil of a step-by-step relay 26 having an armature 2l, pivoted, as at 28, and retained in its normal position against a stop T by a spring S. At one end the armature 21 carries a pawl 29 adapted to rotate a ratchet wheel 3i), the pawl being normally held in engagement with the ratchet 3B by means of a spring 3|, as shown. A friction wheel 32 mounted on a spring 33 may be provided to engage the teeth of the ratchet 30 to prevent over-running thereof. Ratchet 30 is secured to a shaft 34, to one end of which is secured a movable contact arm 35 adapted to selectively engage a series of contacts 36, 3l, 33, 39, 40. For the purpose of resetting the contact arm 35 into its normal position in engagement with contact 36, a spiral spring 4I is provided, one end of which is connected to shaft 34 and having its other end fixed. The contacts 3l, 39 and 40 are connected by means of the circuit shown to the laments of electric lights on whose globes may be painted or otherwise inscribed the designations of transmitting stations, herein indicated as A, C and D. Since the receiver herein illustrated is that of station B, no indicating light for this station is, of course, necessary. A battery 42 is connected to the contact arm 35 to supply current for energizing the filaments of the lights and also to supply current to contact 36 for a purpose to be later described.

For the purpose of releasing the pawl 2.9 upon the disappearance of the carrier, a circuit 43 is provided which includes the plate and cathode of an electron discharge tube 44, a battery 45, and an electromagnet 4B, the control grid of tube 44 being connected to the automatic volume control circuit of receiver R. by the lead 41. Circuit 43 also includes the coil of the relay 48 whose armature 49 is adapted to open and close a circuit connecting the contact 36 and an alarm device herein illustrated as a bell 50.

When the receiver is not locked on any carrier, the contact arm 35 is in engagement with contact 35 and the lbell 50 is unenergized. Shortly after the receiver is locked on a tone modulated carrier by the means heretofore described, the carrier is modulated at the transmitting station with -an audio tone, as for example 800 cycles for a short period, such as one-quarter of a second, to form a dot impulse. This impulse is transferred to the relay 20 through the ltransformer I8 and the tone lter |9 which serves to lter out any extraneous noise components in the modulation frequency, the rectier 22 serving to rectify the current supplied to relay 20. The operation VofV relay 2|) Vserves VtoV close the'circuit through battery 25 and the coil of relay 26, causing the armature 21 of the latter to advance the pawl 29 a distance of one ratchet tooth. This causes the contact arm 35 to engage contact 31 and light the filament of lamp A which thereby provides a visual indication that station A is calling receiver B, the single dot being the identifying impulse associated with station A While contact arm 35 was still in engagement with contact 3B, and as soon as the carrier frequency of station A was received, a negative voltage is supplied to the grid of tube 44 by the automatic volume control action causing the tube 44 to become blocked and the release of armature 49 of relay 48, thereby closing the circuit including a battery 42, contact arm 35, armature 49, and bell 50, causing the bell 59 to give an audible indication that some station is calling receiver B. The operation of the bell 59 continues for a short time until contact arm 35 is moved away from contact 36 by means of the dot impulse received. In the illustrated example, after the receiver is locked on the c-arrier frequency of station C, three dot impulses are transmitted which cause the lpawl 29 to advance the ratchet wheel 30 three teeth and the contact arm 35 into engagement with contact 39 so that the circuit through the filament of light C is closed and the light indicates that station C is calling. At the termination of the communication, upon the carrier wave of the transmitting station being no longer transmitted, current passes through circuit 43 and the coil of the electromagnet 46 which withdraws the pawl 29 from the ratchet 30, whereupon the spring 4| returns the Contact arm 35 into its normal position in engagement with contact 36. It will also be noted that the passage of current through circuit 43 causes the armature of relay 48 to :be drawn away from its righthand contact, thereby maintaining the circuit through the bell open so that the bell does not operate. It will be understood vthat upon station D sending out a series of tour dots, the contact arm 3-5 will be advanced into engagement with contact 40 thereby lighting up lamp D to notify the operator of station B that station D is calling.

In the form of the invention in which the receiver R is `of the scanning type disclosed in my 'co-pending application Serial No. 321,377 and locks on the tirst carrier passed over, whether such carrier is modulated or unmodulated, the method of operation of the indicating device of Fig. 2 is similar to that above described. Upon the reception of the modulated or unmodulated carrier and while the contact arm 35 is in engagement with contact 36, a negativ-e voltage is built up by the AVC device of the receiver which is applied to the grid of tube 44 causing this tube to lbecome blocked. This causes the relay 48 to release its armature 49, thereby causing the bell 50 to operate and give an audible indication that some station is calling. The operation of the bell 50 continues until the contact arm 35 is moved away `from contact 35 by means of a received dot impulse in the manner above described.

The embodiment of the invention shown in Fig. 3 is in general similar to that shown in Fig. 2, corresponding parts in the two gures being indicated by similar reference characters. In this embodiment the relay 48 and bell 50 are omitted and the audio output of the receiver is impressed on contact arm 35 by the lead 5| which is connected to one of the windings of output transformer I8. In the receiver illustrated, representing that of station B, a circuit 52 is shown connected to contacts 31 `and 39 and to ground and which includes the signal reproducing device, herein illustrated as a telephone receiver 53.

In the operation of this embodiment of the invention, if the calling station wishes to communicate with all the receivers in the group, a carrier frequency modulated by the 5000 cycle tone frequency is transmitted. Within a short interval, as `for example, one second, all the receivers will have automatically locked on the transmitted carrier and the sending station then transmits one dot impulse, by also transmitting the 800 cycle modulation for a brief interval. This dot impulse causes the operation of relay 2|, electromagnet and causes pawl 29 to advance ratchet wheel one tooth, thereby causing contact arm to engage contact 31 and the closure of the circuit 52. The operators of all receivers may then listen in on the phones 53 to the communication from station A, thus permitting the operator of station A to issue a general order to all the station operators.

Should the operator of station A wish to set up a communication channel with station B only, he transmits a carrier frequency modulated with the '7000 cycle tone frequency to which the subgroup of receivers including receiver B is responsive. After a short interval, during which the receivers of this sub-group have locked on his carrier frequency, station A transmits 3 dots on the 800 cycle modulated carrier. The impulses thus transmitted cause pawl 29 to advance ratchet wheel 39 three teeth and contact arm 35 to engage Contact 39 thereby completing the circuit through lead 5| and the phones 53 and permitting the operator of B to listen to the communication from A. Since receiver B is the only one of the sub-group whose earphones are connected to the third contact 39, only this particular receiver receives the communication from A. It will be understood that in the illustrated embodiment of the invention, the circuit 52 of receiver A is connected to its contacts 31 and 38, and that this circuit of receiver C is connected to its contacts 31 and 40. At the termination of the communication, upon the carrier wave of the transmitting station being no longer transmitted, current passes through circuit 43 and the coil of electromagnet 46, which withdraws the pawl 29 from ratchet wheel 30, whereupon the spring 4| returns the contact arm 35 into its normal position in engagement with contact 36. While in the illustrated embodiment of the invention, a series of only 5 contacts is shown associated with contact arm 35, it will be understood that any desired number of contacts-may be provided in accordance with the number of receiversin each sub-group.

It will be noted that little provision has been made for preventing the `actuation of the step relay by static impulses, since the apparatus is particularly intended for use at the ultra high frequencies of 100 megacycles and above, where static is seldom if ever encountered. For a more detailed description of the means for causing the receiver to lock on a carrier modulated by a suitable tone frequency, reference is made. to Fig. 4 in which the audio amplifier 8 is shown as comprising the tubes VTI and VTS which are resistance coupled, as shown, the latter tube also being provided with an lextra plate 54 which cooperates With the tube cathode 55 to provide a diode rectifier. The output circuit of vtube'VTl is connected to the earphones I5 by means'of a band pass lter 9. The filter comprises a push pull output transformer 56 which is designed to have a considerable amount of leakage reactance and whose primary winding has a condenser 51 connected across it. By a proper Choice of the value of this condenser, a sharp lcut off above 3000 cycles is obtained so that any tone frequency of 4000 cyclescr higher ceases to be annoying to the user, even when the tone frequency is left continually on during the speech transmission. It is found that a lter of this type will allow the usual range of telephonie speech frequencies to pass and providea rapid attenuation above those limits.

The scanning device I4 comprises the vacuum tubes VT5 and VTS, the plate of each tube being connected to the grid of the opposite tube through the condenser CI and C2 having ya capacity of 0.5 mfd., and the grids of the tubes being connected to their cathodes through the 2.0 megohm resistors R5 and R6, to thereby produce a half cycle sweep voltage of substantially square wave form. This voltage is supplied to the control grid of a control tube VT4v through a lead 58, resistor R'I and condenser C3, the latter serving to block off the direct current voltage.

Resistor R'I and condenser C4 act as a filter to l take voltage of square wave form at the plate of tube VT5 and change it to a substantially sinusoidal Wave form. At the same time the resistor R1, being 10 megohms, reduces the voltage from several hundred volts to i3 volts. vantage in making R'I so large is that it does not destroy the symmetry of the sweep oscillator I4. It will be realized that the half cycle oscillator covers the entire scanning spectrum in one direction completely in one second. A con- Vtrol voltage is also supplied to the control grid of VT4 through resistor R5 and the director line 58 by the director device I5.

The director I5 consists of a director driver tube VTI whose control grid is connected through lead 59 and condenser 65 to the output of the intermediate frequency amplifier 6, the cathode of the tube being connected to ground through the resistors RI4 and RI5 connected in series. The tube cathode is connected to the B supply voltage source through the resistor RI3 and the B supply is also connected through the resistor RIB to the screen grid 6I of the tube VTI and also to the plate of the switching tube VTIU, as shown. The plate circuit of tube VTI comprises the low impedance primary windings 52 and 63 which are coupled, respectively, to the secondary transformer windings of the circuits TCI, TC, one of which is tuned to a frequency of from 5 to 10 kilocycles above the intermediate frequency,

A further adand the `otherof which is. tuned'by the same amount below the intermediate frequency. These circuits feed the diodes VT2 to develop a differ-` ential` voltage across the resistors RI Vand R2. As shown, one end of the resistor RI .is grounded and one end of resistor R2 is connected by director line 58 and resistor R9 to the control grid of tube VT4. l

Theoscillator I2 is shown as comprising the tube VT3 whose cathode is grounded and whose plate circuit is provided with a tuned tank circuit comprising the coil LI and condenser 64. Itis found that by making the condenser 64 of fixed value and varying the inductance of coil yLI so as to change its permeability, as by means of a powdered ferro-magnetic core, as indicated at 55, substantially equal percentage tuning effects are secured throughout the band of frequencies. struction to the core 4 above described.

Consideringthe `circuit which is grounded at the lower Yend of coil LI, the oscillator gridvv receives charging current through L2 representing the mutual inductance of the tickler coil with LI. The grid current at the end of L2 is thus exactly out of phase with the circulating current in Ll. Before reaching the grid Aof tube VT3, the current must pass through some series leakage inductance, represented by L3, which produces a reactance of rather small amount, but which is quite effective due to the very heavy current iiowing through it. At the grid of VT3, energy is supplied which is so phased in relation to that of the plate tank circuit as to cause the tube to oscillate, but is not quite 180 phase angle from the plate. The amount of phasing may be varied as desired, by making L3 of the proper value, even, if necessary, by winding an additional uncoupled coil in series with the grid, or by more loosely coupling L2, and by -making this coil larger we can increase the leakage inductance to any desired amount withoutI having any physical coil, as represented by L3. With this energy, preliminarily phased, as described, the grid energy of tube VT4 may now be further phased in the same direction by the series resistance R8.

For the purpose of controlling the frequency of theoscillations generated by oscillator VT3, the plate of tube VTS is connected to that of a control tube V'I4 and both plates connected to the mixing device5 through a condenser 65. Control tube VT4 is provided with the usual supply voltage, its inner grid being given a normal bias of approximately -6 volts by connecting its cathode to the bleeder resistor 51 at a point approximately this number of volts above ground.

VThis permits the operation of the tube at the mid-point of its control range, as determined by plotting a curveofgrid bias against oscillator frequency which results in an S-shaped curve, so that we secure operation about .the mid-point thereof. The voltage generated by the sweep oscillator I4 is arranged to be a total peak-to-peak of 6 volts so that the effective voltage at the grid of control tube VT4 from the sweep oscillator is i3 volts.v The voltage swing changes the frequency of the oscillator tube VTS by an amount suflicient to sweep the receiver through a range of frequencies approximately '1% of thev carrier frequencyi When the director is rendered operative, in a manner to; ble described, it generates a voltage of :20 volts even on the weakest signal for` which the system is designed. This voltage is suicient to overpower the sweep voltage and This core is preferably of similar coni to tune the receiver tothe incoming carrier within approximately 1 kilocycle, at a carrier frequency of 150 megacycles.

The means for rendering the director operative upon the reception of` a carrier modulated by the proper tone frequency comprises the switching tube VTIO which is energized by the tone lter I'I or II' (Fig. 1). This filter comprises an amplifying tube VT8 whose grid is conneoted to the audio amplifier through a small condenser CI and the coupled resonant circuits TC3 and TC4 which may be permeability tuned to the desired tone frequency, as indicated at 5B. Where the identifying tone frequency is within the preferred range of from 4000 to 20,000 cycles, each of these circuits is tuned to that frequency.

The selected tone frequency voltage is amplified by tube VT8 in a resistance coupled stage and rectified by the circuit including the diode 54, 55 and resistor RII, one end of which is grounded. The grid of the audio output tube VT9 is normally biased to -10 volts by means of the self-biasing resistor G9 which is shunted by a condenser l'10. This arrangement provides a normal negative voltage of -10 volts on the plate 54 of the diode with reference to its cathode 55. A signal must, therefore, develop a potential on the positive peaks in excess of the 10 volts negative bias in order to permit the diode to rectify. A weak desired signal which may, for example, be modulated at a tone frequency of 5000 cycles for purposes of station identification, be scanned by the action of the oscillator I2, control device I3 and sweep oscillator I4, and in about 3 milliseconds builds up in the resonant circuits TG3 and TC4 to approximately full amplitude. This voltage is amplied by tube VT8 and impresses substantially 20 volts positive peak voltage on the diode plate 54, thus overcoming its 10 volts negative bias and producing a rectified output voltage across RII of the order of 10 volts. This voltage is filtered by the filter circuit comprising resistor RI2 and C8 and will be hereafter referred to as the switch operating voltage.

It is assured that no director voltage is developed when the receiver is passing through the extremely strong undesired carrier by maintaining the screen 6I at a potential of from -20 to 30 volts, which is sufficient to absolutely block the tube VTIO so that no director voltage is developed in the circuits TC I, TC2. The screen 6I is connected to the plate of the tube VTI!) which, being unbiased, has a very low resistance and draws such a large current through RIB that the screen voltage is about 20 volts above ground. Since the cathode of this tube remains at substantially 50 volts above ground, the screen voltage is 30 volts negative with respect to the cathode.

When a desired signal is encountered and current of the tone frequency builds up the switch operating voltage as before described, this voltage is negative due to the passage of current through the diode 54, 55 and resistor RI I. This negative voltage being applied to the grid of tube VTIIJ is suflicient to block this tube with the result that the iiow of current through resistor RII:` is considerably decreased and the voltage of screen 6I immediately rises to about 150 volts above ground and VTI is then operative. The carrier wave of a desired station now being received, resonant voltages are built up in the circuit TCI and TG2, the differential voltage developed being applied to the grid of the control tube VT4. This voltage is sucient to override the sweep voltage developed by the sweep oscillator I4 and lock the receiver on the carrier. The noise voltages appearing in the director line 58 are preferably filtered out by the filter circuit which comprises the internal resistance of the director device which is about 2 megohms, and the condenser C4 which has a capacity of 0.5 microfarad.

It is to be noted that both the tone and the carrier must be present to maintain this condition. If either one disappears for more than a few milliseconds, the director driver tube VTI becomes blocked so that no director voltage is developed and the receiver instantly resumes the scanning condition under the control of the sweep oscillator I4.

Should a very strong undesired carrier be encountered having, for example, 6000 cycles modulation on it, the requisite switch operating voltage will not be developed by the resonant circuits TC3, TC4.

The voltage developed by the circuits TO3, TC4 as determined from their combined selectivity curves at 6000 cycles is only about one-tenth of that developed at their resonant frequency of 5000 cycles. But this voltage difference in itself would be insufcient to prevent the development of an effective switch operating voltage which might lock the receiver on an undesired carrier which may be 100,000 times stronger. One of the means which has been found effective to prevent the development of the switch operating voltage for an undesired carrier comprises the tube VTI, the constants of this tube and its associated circuits being designed so that the maximum power developed in its plate circuit is substantially milliwatts. This amount of energy is insufficient to develop more than about '7 peak volts across the diode 54, 55 when a very strong signal` modulated at 6000 cycles is encountered since to get any substantial response from a resonant circuit olf its resonant frequency, a considerable amount of power is required to drive it. The 7 volts developed across the diode is 3 volts less than the minimum voltage required to develop any direct current voltage across RII so that no output voltage whatsoever is developed in the diode circuit and the tone switch is not operated to render the director effective to lock the receiver on the 6000 cycle modulated carrier.

When a very strong undesired carrier is swept over, a large transient voltage is developed as the slope of the resonance cur've of the receiver slides past the carrier frequency. This strong transient does, by shock excitation, cause the oscillation of circuits TC3, TC4 at their own resonant frequency, but due to the extremely limited driving power provided in the output circuit of tube VT'I, only about 5 volts is generated across the circuit TC4 after amplification by VT8 which is, of course, insufficient to overcome the biasing voltage of diode 54, 55 so that no switch operating voltage is developed.

The sweep oscillator I4 and its associated filter circuit comprising RI and C4 illustrated develops a substantially sine shaped voltage wave to effect the scanning action, but it will be understood that a source which generates a voltage wave of other desired shapes such as triangular, sawtooth, etc., may be used instead.

It has been noted after considerable experience with receivers of this type that as the frequency is raised we can attain such a frequency that the ordinary noises which prove so troublesome in `ordinary reception, such as static, diathermy machines, sparking commutator motors and ignition noises, gradually die out, and at some vague point, probably below 150 megacycles, become extremely rare in a sharp receiver. There is left, of course, the thermal agitation noise in the circuits and the noise and shot effects in the tubes. This is a more or less regular hiss type of noise of quite a denite and dependable character1 and is easily recognized by any one skilled in the art.

Where space and weight are no consideration in designing a receiver of this type, we should problably always prefer a definite sweep voltage such as that produced by the 0.5 cycle oscillator shown in Figure 4, but experience has shown that if the switch SW2 be closed, the fluctuating noise voltages produced in the output of the audio stage VTI by the amplification of the thermal agitation and shot eect current of the rlrst stage is suilicient to sweep the control tube through its entire range quite rapidly and at random, but in a given period we find we cover the entire useful range of sweep in less than a second, although it is recognized that several pulses of the same polarity may follow each other rather thanthe smooth pulses of always alternate polarity which we obtain from the more disciplined action of the 0.5 cycle oscillator, which is, of course, omitted under this condition. The above condition obtains only when the receiver is of great sensitivity, i. e., so sensitive as to develop on peaks a voltage sufficient to fully control the control tube.

In a receiver designed for operation over a tuning range of from 100 to 200 megacycles the following circuit constants are found suitable.

The general B-I- supply is 250 volts. Vacuum tubes of the following types are found suitable for use with the circuits described:

VTI=type 6J7 pentode VT2=type SHG double diode AVT3=type 955 acorn triode VTlI--type 954 acorn pentode VT5, VT6=type 6N7 double triode VT'I, VT8=type 6N7 double triode In using the receiver, the operator tunes the input circuit of the radio frequency amplifier 3 so that it will pass the carrier frequency or frequencies of the transmitter or transmitters with which it is desired to establishcontact. In the illustrated embodiment this tuning is effected by manual adjustment of the permeability tuning means 4 which is unioontrolled in any known manner with the tuning means B5 of the oscillator. Since the frequency of oscillator VTS is being cyclically varied through the action of the control tube VT4 whose reactance isin turn being cyclically varied by the sweep oscillator VT5, VTS, as soon as a carrier appears which forms a beat frequency with the oscillator frequency which lies within the frequency band passed by the intermediate frequency amplier E, an energizing voltage is applied through lead 59 to the control grid of the director driver tube VTI. If the received carried is not modulated by the proper tone frequency, the tube VTI isblocked in the manner above explained and no director voltage is built up in the circuits TCI, TG2, so that the receiver is not locked on the carrier, but the scanning action is continued under the control of the sweep oscillator III.V

Upon receiving a desired carrier which is modulated with the tone frequency to which the tone lter circuits TG3., TCI! are resonant, a tone switch operating voltage is developed across the diode resistor RI I which is suiiicient to substantially block the tube VTIIl. This action causes the voltage of screen'SI to rise very substantially, so that the plate current of VTI energizes the director VT2, and a minimum director voltage of $20 volts is developed and through line 58 is applied to the control grid of the control tube VT4. This high voltage overpowers the control voltage of i3 volts developed by the sweep oscillator and serves to maintain the reactance of the controlk tube at a substantially constant value, thereby maintaining the oscillator frequency at a substantially constant value which heterodynes with the carrier frequency to produce the proper beat frequency which lies within the band of frequencies passed by the intermediate frequency amplifier 6. The receiver remains locked on the carrier as long as the tone modulated carrier is received. If the carrier disappears, no voltage is supplied through line 59 to energize thev director with the result that no director voltage is developed and the receiver resumes the scanning condition under the control of the sweep oscillator M. If the carrier continues to be received but the tone modulation disappears, no currents are available of the tone modulation frequencyin the output circuit of tube VT'I, so that no switch operating voltage is developed by the tone lter. This causes the tube VTI i) to draw a heavy plate current, and the voltage of screen 6I to drop low enough to block the director driver tube VTI. Since no director voltage is now developed or applied to the control grid of the control tube, the sweep oscillator resumes control of the control tube and the scanning operation is resumed.

When a receiver of this type is to be used in point-to-point communication where there is no possibility that the carrier wave will seriously weaken once contact has been established, it is perfectly feasible to operate the receiver in the tone lock, carrier hold condition. In this condition the tone is impressed on the carrierl only for a second or so to be sure the receiver has had time to lock on the designated carrier, and the tone is then removed leaving the receiver locked on the carrier. Means for doing that include the connection 'II and the switch SWI. Upon closure of this switch the receiver is placed in the tone lock, carrier hold condition, since any negative voltage developed across RI will be applied to the grid of VTIEI, the switch tube.

Assuming a receiver to have scanned its usual portion of the spectrum, and encountered a carrier having a proper tone to which TC3 and TCII are tuned, the necessary switch operating voltage will appear across RII and be applied to VTIll. While the receiver is scanning, of course, VTI is blocked and no voltage can appear across RI. However, once VTI IJ switch tube has been blocked, VTI in turn is unblocked and a negative voltage builds up across RI due to the carrier wave. This negative voltage is immediately applied to the grid of VTIO through the connection 'Il and is of sufficient amplitude to maintain VTIU in a fully blocked condition. If the tone is now removed from the carrier, the voltage from RI will hold the system in operative condition until such time as the carrier wave disappears. It is obvious that once the carrier disappears, the receiver resumes the scanning condition and will again lock only when a carrier identified by the proper tone is encountered.

Whiley the receiver shown in Fig. 1 and above described is a single superheterodyne, the apparatus in which the invention was developed was a double detection type superheterodyne with a first intermediate frequency of 17.5 megacycles and a nal intermediate frequency of 0.46 megacycle.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined Ytothe embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. A radio receiver comprising, in combination, a signal reproducing means, an input portion receptive to signal modulated carrier waves of a single predetermined frequency transmitted from any one of a plurality of transmitting stations and indicating means other than said signal reproducing means coupled to said receiver and responsive to modulations other than the signal modulations on the received carrier for indicating the identity of the transmitting station, said indicating means comprising a member selectively adjustable between an initial position and a plurality of terminal positions and means responsive to the disappearance of the carrier for automatically returning said member from a terminal position to its initial position.

2. A radio receiver having an input portion receptive to signal modulated carrier waves of a single predetermined frequency transmitted from any one of a plurality of transmitting stations, indicating means coupled to said receiver and responsive to modulations on the received carrier for indicating the identity of the transmitting station, an alarm device, means connected to said receiver and responsive to a received carrier for causing the operation of the alarm device, and means responsive to the disappearance of the carrier for restoring the indicating means to its initial condition.

3. In combination, a radio receiver having an input portion receptive to signal modulated carrier waves of a predetermined frequency transmitted from any one of a plurality of transmitting stations, a signal reproducing device responsive to the signal modulations continually coupled to the output of said receiver, a sound producing device, means connected to said receiver to automatically cause said sound producing de- Vice to become operative upon the reception of carrier current in the input portion of the receiver, a tone filter electrically connected to said receiver and means responsive to the passage of current through said tone filter to automatically render said sound producing device inoperative.

4. A radio receiver adapted to receive speech modulated carrier waves, said receiver comprising in combination, a translating device for reproducing the speech modulations, a movable switch arm, and a circuit arranged vto connect the output of the receiver to said translating device, said circuit including a portion having one end thereof continuously connected to the output of the receiver and its other end to said movable sWitch arm, said switch arm being normally positioned so as to disconnect the translating device from the output of the receiver and means connected to the receiver and responsive to a single tone modulated carrier impulse for moving the switch arm from its normal position into position to connect the translating device to the receiver.

5. A radio receiver adapted to receive signal modulated carrier waves and having an output circuit, a translating device arranged to reproduce the signal modulations, a movable member connected to said output circuit and adjustable between an initial position and at least two other positions, a circuit arranged to connect the translating device to said movable member in response to the adjustment of the movable memberinto either of said otherV positions and means responsive to modulations on a received carrier for selectively adjusting the movable member from its initial position to either of said other positions.

6. A radio receiver adapted to receive signal modulated carrier waves comprising, in combination, a circuit tunable over a range of frequencies, means for automatically tuning said circuit over the frequency range, means responsive to the reception of modulation on a carrier having a frequency within said range for rendering ineifective the action of said automatic means on the tuning of said circuit, a signal reproducing device and means responsive to modulations other than the signal modulations on a received carrier for selectively causing the signal reproducing device to remain inoperative or to cause it to reproduce the signal modulations on the carrier.

'7. A radio receiver of the superheterodyne type adapted to receive signal and tone modulated carrier waves, said receiver comprising, in combination, a rst detector, an oscillator, means for automatically varying the frequency of the currents generated by said oscillator, means responsive to the tone modulations on a received carrier for rendering ineifective the action of said automatic means on the oscillator, a signal reproducing device and means responsive to modulations other than the signal modulations on a received carrier for selectively causing the signal reproducing device to remain inoperative or to cause it to reproduce the signal modulations.

8. A radio receiving system for receiving signal and tone modulated carrier waves comprising at least two radio receivers of the superheterodyne type, each of said receivers comprising, in combination, a rst detector, an oscillator, means for automatically varying the frequency of the currents generated by said oscillator, means responsive to the tone modulation on a received carrier for rendering ineffective the action of said automatic means on the oscillator and a signal reproducing device; means responsive to a predetermined number of modulations on a received carrier for causing the signal reproducing devices of all the receivers to reproduce the signal modulations on the carrier and means responsive to a different number of modulations on the carrier for causing the signal reproducing device of only one receiver to reproduce the signal modulations on the carrier.

9. A radio receiver adapted to receive signal modulated carrier waves and having an output circuit, a translating device arranged to reproduce the signal modulations, a movable member connected to said output circuit and adjustable between an initial position and at least two other lating device to said movable member in response to the adjustment of the movable member into either of said other positions, means responsive to modulations on a received carrier for selectively adjusting the movable member from its initial position into either of said other positions and means for automatically returning the movable member to its initial position in response to the disappearance of the carrier.

10. A radio receiver comprising, in combination, a signal reproducing means, an input portion receptive to signal modulated carrier Waves of a predetermined frequency transmitted from any one of a plurality of transmitting stations, a plurality of spaced contacts, an alarm device positions, a circuit arranged to connect the transconnected to a rst of said contacts, an indieating device other than the signal reproducing means connected to a second of said contacts and adapted to indicate the identity of a transmitting station, a switch blade movable along said contacts and normally positioned on said rst contact, means responsive to the reception of a carrier Wave to automatically cause the operation of said alarm device While said switch blade is on said first contact and means responsive to modulations on the carrier to advance said switch blade from said rst Contact onto said second contact to thereby cause the operation of said indicating device.

SIDNEY Y. WHITE. 

